Radio Frequency Identification (“RFID”) is a generic term for technologies that use radio waves to automatically identify individual items. Objects can be identified using RFID by storing a serial number that identifies the object on a chip that is attached to an antenna. The chip and the antenna together are called an RFID tag. An RFID reader sends out electromagnetic waves that are received by the antenna on the RFID tag. Passive RFID tags draw power from this electromagnetic field to power the chip. Active tags use their own batteries to power the chip. The tag responds to the reader by transmitting a bit stream to the reader that contains information about the tag (serial number, etc.). The current state of RFID technology is described by [1] K. Finkenzeller in “RFID Handbook” (John Wiley & Sons, 2003).
RFID systems operate at many different frequencies. The most common are low frequencies around 135 KHz, high frequencies around 13.56 MHz, ultra-high frequencies around 900 MHz, and microwave frequencies around 2.45 GHz and 5.8 GHz.
Current RFID systems are not suited for precise location of tags because readers transmit very broad beams that cause tags in a large region to respond. Moreover, when two or more tags respond simultaneously, the transmissions from the tags get scrambled and become unintelligible to the reader. This phenomenon is known as tag collision. Procedures that involve repeated transmissions of tag data have been developed to deal with tag collisions [1, Chapter 7]. However, the interrogation speed (number of tags interrogated per second) is reduced when a large number of repeated transmissions are needed, so it is desirable to reduce tag collisions as much as possible.
The RFID reader's efficiency is related to its coverage or “accuracy,” which is measured by the percentage of tags within range that are read correctly. The accuracy of today's readers is not acceptable for many applications, which require 100 percent accuracy. For example, a study published in the article “Smart Tags for Your Supply Chain,” McKinsey Quarterly, 2003, Number 4, found that RFID-tagged pallets failed 3 percent of the time even when double-tagged, and only 78 percent of the individually tagged pallets were read accurately.
According to the article “RFID will present a stiff test,” published in Supply Chain Management Review, Jan. 15, 2004, the main cause of low reader accuracy is the inability of readers to transmit enough power to activate tags that are surrounded by other objects such as tags affixed to items stored in the middle of a pallet. The article reports that ad hoc repositioning of the RFID tags or increasing reader power can often fix this problem.
The problem of reader collisions is another barrier to the large-scale deployment of RFID. Reader collisions can occur when the interrogation zones of two or more readers overlap. In the article “Why UHF RFID Systems Won't Scale,” RFID Journal, July 2004, H. L. van Eeden states that “The main technical problem facing end-user companies is the possibility of large-scale reader interference that could render UHF RFID installations completely inoperable and severely limit the rollout of UHF RFID systems.”
The problems of reader collision and low reader accuracy are related: if one attempts to solve the problem of low reader accuracy by increasing the reader power, then the interrogation zones grow and reader collisions become more frequent.
The following five U.S. Provisional Applications describe RFID readers that transmit data signals that cause the tags to respond and scramble signals that do not cause the tags to respond: [2] “Method and apparatus for secure transmission of data using array,” U.S. Provisional Application No. 60/550,355, filed Mar. 5, 2004, [3] “Method and apparatus for preventing unauthorized transmitters from gaining access to a wireless network,” U.S. Provisional Application No. 60/550,411, filed Mar. 5, 2004, [4] “Method and apparatus for precise location of RFID tags,” U.S. Provisional Application No. 60/561,433, filed Apr. 12, 2004, [5] “Optically guided reader of RFID tags,” U.S. Provisional Application No. 60/603,531, filed Aug. 20, 2004, and [6] “Method and apparatus for improving the efficiency of RFID systems,” U.S. Provisional Applications No. 60/613,428, filed Sep. 27, 2004. These five provisional applications are incorporated herein by reference in their entirety.
The data and scramble signals are transmitted with different beams that are adjusted such that the scramble signals overshadow the data signals in all but selected regions. Hence, a tag will respond only if it is located in one of the selected regions, called the interrogation zones.
Provisional patent application [2] describes methods for using sum and difference patterns of array antennas to transmit data into selected narrow angular regions. The data signal is shielded by a scramble signal that makes the total transmitted signal unintelligible everywhere except in the narrow angular region. The scramble signal is also allowed to contain its own data that is different from the data carried by the data signal. Provisional patent application [2] further describes how the precise angular positions of RFID tags can be determined. Provisional patent application [4] describes how the width of the interrogation zone can be reduced and how the absolute location of a tag can be obtained from triangulation. Provisional patent application [5] describes how the interrogation zone can be visualized with optical sources. Provisional patent application [6] describes how the efficiency of RFID readers and reader networks can be improved through measurements, modeling, and inversion.